This invention relates generally to beds, or reservoir layers, and in particular to methods for determining the net and gross thickness of a bed. Hydrocarbon reservoirs in general are not perfect blocky beds, but rather have internal structure in the form of embedded layers of non-reservoir or poor reservoir quality material. However, existing methods for determining gross or net reservoir thickness of these reservoirs ignore or do not accurately consider the internal structure. Thus, those methods have neglected to include effects associated with the internal structure of beds, in the determination of the net and gross thickness of beds.